Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae. Seaweeds can also be classified by use (as food, medicine, fertilizer, industrial, etc.).

Most people know two general categories of seaweeds: wracks (members of the brown algal order Fucales such as Fucus) and kelps (members of the brown algal order Laminariales such as Laminaria), and many have heard of Carrageen or Irish Moss (usually a red alga, Chondrus crispus) and Dulse or Dillisk (also a red alga, Palmaria palmata). Seaweeds make up the Sargasso Sea, a large ocean gyre in the western Atlantic where drift plants of several species of the genus Sargassum accumulate. Seaweeds are particularly important ecologically: they dominate the rocky intertidal in most oceans, and in temperate and polar regions cover rock surfaces in the shallow subtidal. Although only penetrating to 8-40 m in most oceans, some are found to depths of 250 m in particularly clear waters (Mediterranean, Caribbean, Brazil). The Giant Kelp (Macrocystis) is one of the largest plants in the world, which in western North America forms an important association with the newly rescued Sea Otter.